The present invention relates generally to cellular basestations and more particularly to full duplex simultaneous (In Time) and overlapping (In Space) wireless transmission and reception on the same frequency band.
The transmission and reception in current commercial wireless communication systems occur in orthogonal resource blocks (RB) where a resource block indicates a specific partition of available frequency resources in space and time. Two resource blocks are orthogonal if they either differ in frequency, time or space or a combination of these. In particular, cellular systems are designed such that uplink and downlink transmissions take place in orthogonal time, i.e., time division duplex (TDD) systems, or in different frequencies, i.e., frequency division duplex (FDD) systems. In conventional cellular systems there is no direct communication among the users or among the base stations over the air, therefore, there are only two communication modes of interest: the downlink transmission from a base-station to a user or the uplink transmission from a user to the base-station.
More recent wireless communication system designs consider the use of relays to improve the transmission range, rate, and reliability. However, still orthogonal resource blocks have to be used for the transmission and reception from the relays. Systems that work under the orthogonality constraints on the resource blocks for transmission and reception from any of the network terminals, e.g., the base stations, relays, users etc. are deemed to work in the half duplex (HD) mode.
In contrast, a full duplex (FD) system is one in which the terminals are able to transmit and receive on the same resource blocks, i.e., a terminal can transmit and receive in the same frequency band at the same time (and of course the same space, as the transmit and receive antennas on the terminal, in general, are close to each other). If a FD is enabled, then many system functions have to be redesigned including the design of control signals, physical layer schemes such as relaying, and MAC and upper layer signaling and operations such as scheduling. The simplest and most direct benefit of a FD enabled system, however, is doubling of the aggregate uplink-downlink capacity. A FD enabled wireless system would add to the spectral efficiency per area (eg. bits/second/Hz/meter2) enabled by newer technologies such as relays and femto-cells. Also, the use of multiple antenna transmission in FD systems would result in at least the same benefits as in HD systems. Other benefits of FD transmission include the elimination of frequency guard band between the uplink and downlink frequency band found in FDD systems and the elimination of uplink-downlink synchronization found in TDD systems.
There are a number of well-known problems in ad-hoc networks such as the Deafness problem, the Hidden terminal problem etc., that can be easily solved when full duplex communication is enabled. Moreover, in CSMA networks it is possible to improve collision avoidance and collision notification schemes which in turn would improve effective network capacity. In these cases, the net effect would be more than just the doubling of the aggregate transmission/reception rate at any particular terminal.
The main obstacle to FD implementation is the lack of availability of practical designs rather than any inherent physical limitation. In theory, since the terminal is (non-causally) fully aware of the local signal to be transmitted, it should be able to subtract it from the received signal. However, several factors affect the implementation of such a solution. First, at a terminal, the local transmission from the transmit antenna and the reception at the received antenna are affected by the antenna pattern. Second, the received signals have traveled through the air (from another terminal) and are attenuated at least as much as the free-space loss. Thus, exact recreation of the signal intended to be received at the receiver (we will refer to this as the “intended receive signal” from now on) might not be possible or very hard. Third, what aggravates the problem even more is the power ratio between the locally transmitted signal (which is self interfering) and the received signal, which is usually large (ie, the locally transmitted signal power could be several orders of magnitudes larger than the received signal power).
Therefore, even a slight deviation in the device functions of the devices that are used to subtract the locally transmitted (self interfering) signal from the received signal, would result in a considerable residue of the interfering signal power to leak and remain combined with the received signal; this may mask out the intended receive signal, completely. We further discuss this issue in more detail later in this paper. To achieve a better precision (than just radio-frequency level subtraction) one may think of using digital processing. However, the digitization noise would be more or at least comparable with the intended receive signal which could make the recovery of this signal impossible. This problem is somewhat similar to the reception problem in satellite communications where the received signal is usually very close to the noise floor. In this case, even with HD transmission, it becomes hard to identify the intended receive signal.
Recent works have considered the feasibility of full duplex (FD) wireless communications in practice. While the first FD system by one person relied on a specific antenna cancellation technique to achieve a significant portion of self-interference cancellation, the various limitations of this technique prompted latter works to move away from antenna cancellation and rely on analog cancellation achieved through channel estimation. However, the latter systems in turn require the use of variable attenuator and delay elements that need to be automatically tuned to compensate for the self-interference channel. This not only adds complexity to the overall system but also makes the performance sensitive to wide-band channels. More importantly, none of the existing FD schemes can be readily scaled to MIMO systems where the nodes have more than two antennas.
By enabling full duplex communication we can simultaneously send in the uplink and downlink and it could mean doubling the use of spectrum, See FIG. 1. In half-duplex systems we either receive or transmit in time TDD or in frequency FDD, so it may be thought that we waste half of the resources.
The main challenge of the full duplex communication is to cancel the self interference that is orders of magnitude stronger than the received signal from the intended transmitters. However this interference is partly known due to the fact that the transmitter exactly knows its own transmitted signal however the exact channel between the transmit and receive antennas at the base station is not known. It would be even worse if this channel is time varying (fading) because we then need to estimate this channel more frequently. FIG. 2 shows the strong self interference in comparison to the weak received signal from a mobile station or user.
There are two possible deployments of the full duplex communication with respect to a fixed number of transmit and receive RF chains. In practice the main complexity involved with the use of multiple antenna systems is associated with the number of RF chain due to the fact that channel estimation, precoding, beamforming, multiple stream transmission, and demodulation all depends on the number of receive RF chains or transmit RF chains. Depending on if we use one antenna for each pair of the transmit RF chain and receive RF chain or if we use two antenna one for receive RF-chain and one for transmit RF-chains. we can have one of the two possible deployment scenarios. Please see FIG. 3.
While both systems might have marginal pros or cons in half-duplex systems, Applicants strongly advocate the use of one antenna per RF chain for full duplex communication because it does not change the system complexity. The cost associated with using more physical antennas well worth the possible gain that can be achieved by this deployment scenario. In the sequel, we provide methods for both systems and in particular we address how to allocate the antennas for either transmit or receive if the other deployment scenario is used.
A full-duplex wireless device that can transmit and receive at the same time in the same frequency band by definition would need at least one Tx and one Rx antenna. The key challenge in realizing such a device lies in addressing the self-interference generated by the Tx antenna at the Rx antenna. As an example, consider a WiFi signal with a transmit power of 20 dBm. A Tx-Rx antenna separation of about 6-8 inches results in a path loss of about 40 dBm (depending on channel characteristics), resulting in a self-interference of at least −20 dBm. With a noise floor around −93 dBm, one would further require a self-interference cancellation of at least 73 dB to be able to decode the desired received signal. While one can solely employ digital interference cancellation techniques, current ADC's do not have a resolution to pass a received signal which is 73 dB less than the noise floor. Hence, several practical full duplex (FD) systems \cite{choi-mobicom'10,jain-mobicom'11,melissa-asilomar'10} have been proposed that couple RF cancellation along with digital cancellation to achieve the desired level of self-interference suppression.
A prior known work proposed an architecture that used a combination of RF cancellation and digital cancellation techniques. RF cancellation included both antenna as well as analog cancellation (using noise cancellation circuits), contributing around 30 dB and 20 dB of cancellation respectively. With an additional 10 dB from digital cancellation, this resulted in a total of 60 dB suppression. Although not sufficient for WiFi, this was sufficient to enable FD communication in 802.15.4 systems (with 0 dBm transmit power). Antenna cancellation was achieved with the help of two Tx antennas (three in total) being placed at d and d+λ/2 distance from the Rx antennas. The λ/2 adds a phase shift of π to one of the transmitted signals to help cancel the other transmitted signal at the Rx antenna.
Three limitations of such an antenna cancellation approach were pointed out in previous works: (i) the dependence on λ allows for maximum cancellation only at the center frequency, with performance degrading for frequencies away from the center—a problem for wideband systems; (ii) employing an additional antenna may not justify the gains compared to a 3×3 MIMO system, and (iii) due to asymmetric antenna placement, manual tuning of amplitude and phase of the closer Tx antenna is required to achieve a null, which prevents real-time operation.
The first limitation is that the design required the placement of one of the Tx antennas at a distance d+λ/2 which then depends on the bandwidth and thus leading to maximum cancellation only at the center frequency (and hence not efficient for wideband signals). Such placement was dictated by the need to create a π phase shift between the two Tx signals and could have been enabled through a phase shifter internal to the device which would then have led to a symmetric antenna placement. The authors of that work argued against such a design under the assumption that it will create many null points at the far-field. But this assumption was based on a free-space path loss model for the far-field while in reality the far-field in general should be modeled under the Raleigh fading model; thus there should be no more nulls in the far-field with symmetric placement of antennas than asymmetric placements.
The second limitation is that the design requires an extra antenna which may not be justifiable compared to a 3×3 MIMO system. MIMO transmissions require each antenna to have a Tx/Rx chain which is not the case here; antenna cancellation merely requires an extra passive antenna element together with a fixed phase shifter and the overhead is not comparable to a MIMO system. If at all any comparison should be made, it should be with a 2×2 MIMO system. Even a single antenna system would require two communications processing chains, one for Tx and one for Rx; thus a comparison with a 2×2 MIMO system may not be justifiable either.
The third limitation is that the design requires manual tuning of variable attenuators and phase shifters to compensate for channel changes (even if channels between the two Tx antennas and the Rx antenna are symmetric, the channels will still change with time); such requirement is primarily due to the use of asymmetric positioning of the antennas (see the first limitation above) and thus disappears when antennas are symmetrically placed.
One other limitation Applicant sees with antenna cancellation using asymmetric placement antennas is that it is not apparent how it can scale to MIMO systems. Although one might envision an extension using two Tx and one Rx antenna for every transmitted/received MIMO stream, this would require antennas to be placed such that each of the Tx pairs lead to self-interference signals which are 180 degrees out of phase at every Rx antenna.
To avoid the above limitations, the authors moved away from antenna cancellation and instead proposed the use of a two antenna (one Tx, one Rx) in their subsequent scheme, where a form of analog (BALUN—balanced to unbalanced transformer) cancellation was used.
A BALUN element acts as a π phase shifter, which was shown to have a better frequency response over a wideband compared to the λ dependent phase shift created using an asymmetric antenna placement. BALUN cancellation was shown to yield 40-45 dB of cancellation; this coupled with 30 dB cancellation using digital cancellation provided the desired level of self-interference suppression for WiFi signals. However, such a design encounters the following limitation. While a BALUN element can create a negative copy of the transmitted signal that can be applied internally to cancel self-interference, one also needs to account for the wireless channel between the Tx and Rx antennas. For this reason, a variable attenuator and delay element are also needed on the path, which in turn have to be auto-tuned and adapted to track the self-interference channel. This not only makes the design complicated but also the performance quite sensitive to wide-band channels. Although with manual tuning it is shown that 40-45 dB cancellation could be achieved, in practice, auto-tuning leads to only a 20 dB cancellation. Other works on FD implementations also do not consider antenna cancellation but consider hybrid schemes where an estimate of the self-interference signal in the digital domain is combined with a negated copy of the transmitted signal in the analog domain to achieve cancellation. This along with digital cancellation was shown to yield only about 35 dB of cancellation, falling short of the desired target.
Next generation wireless devices (access points, base stations, etc.) are expected to be equipped with multiple antennas (more than two). Hence, it is important to design a FD scheme that can co-exist with MIMO. Applicants observe that existing antenna cancellation and analog cancellation approaches cannot be readily extended to MIMO systems. Although one might envision an extension of using two Tx and one Rx antenna for every transmitted/received MIMO stream, this would require antennas to be placed such that each of the Tx pairs (for each stream) lead to self-interference signals which are 180 degrees out of phase at every Rx antenna. However, such an antenna placement cannot be realized for a MIMO system using the prior approach. On the other hand, analog cancellation discussed hereinabove, when extended to N stream MIMO, will potentially require one to estimate the self-interference channel between every pair of N2 Tx-Rx antennas. This in turn results in the use of N2 variable delays and attenuators, each of which has to be auto tuned and adapted to track the N2 self-interference channels, which seems practically infeasible.
Based on Applicant's observations on the limitations of existing FD schemes, antenna cancellation using asymmetric antenna placement as in a prior work and our observation on the limitations of the two antenna scheme proposed already, Applicants propose and justify antenna cancellation with symmetric placement of antennas be considered as a primary RF cancellation technique for providing self-interference cancellation. Specifically, for a single stream transmission Applicants propose antenna cancellation with a symmetric placement of either two Rx antennas and one Tx antenna (which Applicants refer to as Rx antenna cancellation), or two Tx antennas and one Rx antenna (which Applicants refer to as Tx antenna cancellation), each of which is a dual of the other. Applicants show that this design could provide large self-interference cancellation with the following advantages: 1) It leads to the possibility of a two-level design where Tx antenna cancellation is followed by a Rx antenna cancellation with the theoretical potential to double the antenna cancellation gains because of its additive nature. 2) The design scales very easily to MIMO systems which would then enable the co-existence of MIMO with FD. 3) The design could potentially eliminate the need for any other form of analog cancellation which seems limited in practice due to the need for variable attenuators and delay elements and its subsequent lack of scalability to MIMO systems.
Accordingly, there is a need for full duplex simultaneous and overlapping wireless transmission/reception on the same frequency band.